Subionospheric VLF measurements of the effects of geomagnetic storms on the mid-latitude D-region

We examine the effects on the mid-latitude D-region of two geomagnetic storms, the "Halloween storm" of late October 2003 and the 07 April 2000 storm, by means of the associated perturbations of several subionospheric VLF/LF signals propagating in both the northern and southern hemispheres. We use VLF/LF nighttime data from the Holographic Array for Ionospheric/Lightning Research (HAIL), located in the United States (L = 2−3), as well as data from Palmer Station, Antarctica (L = 2.4). On 07 April 2000, a ~ 5 dB depression in VLF amplitudes was recorded at multiple HAIL stations, with a depression onset that occurred later for VLF/LF signal paths at lower latitudes. On both 07 April 2000 and 31 Oct 2003, fluctuations in the amplitude of the VLF signals were first observed in the premidnight sector and persisted through the end of the datarecording period (dawn). The frequency content of the fluctuations was predominantly in the 0.01 to 0.02 Hz range, but extended up to ~ 0.03 Hz. Increases in the energetic electron flux in the loss cone as measured by the NOAA-POES satellites were observed on both 07 April 2000 and 31 October 2003. We suggest that both the signal depressions and subsequent fluctuations were associated with variations in the precipitation flux of energetic electrons onto the upper atmosphere. We provide evidence that the fluctuations and the signal depression coincide with the equatorward edge of the auroral oval extending over the perturbed VLF/LF Great Circle Paths. Quantitative modeling of subionospheric VLF wave propagation incorporating energetic electron flux measurements (and the associated altitude profiles of secondary ionization produced) yield results consistent with the variations in the VLF signal amplitude observed. The occurrence rate of lightning-induced electron precipitation (LEP) events, as recorded on several VLF/LF signals, was seen to be highly variable with geomagnetic activity. Comparison of LEP event occurrence rates with measurements of energetic electron flux levels from the NOAA-POES satellite supports the notion that this variability is largely due to geomagnetic storm-associated increases in the energetic electron population in the slot region.